This is perhaps Hume's most pernicious doctrine. He reduces reason to merely instrumental reasoning—calculating means to ends—and denies that reason can determine the will directly. But this misunderstands the nature of practical reason. Pure practical reason can and does determine the will through the moral law. The categorical imperative is not based on desire or passion but on reason alone. When I recognize a moral duty, reason itself provides the motive for action, independently of any inclination. Hume's account makes morality impossible by making all action ultimately passionate or appetitive. He confuses practical reason with theoretical reason applied to practical matters, missing the autonomous legislative power of practical reason that constitutes the will as free.

Hume's claim that reason is the slave of the passions is both false and dangerous. Reason perceives moral truths—that justice is obligatory, that promises should be kept, that we ought to do good and avoid evil. These rational perceptions provide motives for action independently of passion. A person can do what is right despite their passions urging the contrary, and this is not irrational but the highest exercise of rationality. Hume's error stems from his narrow conception of reason as concerned only with truth and falsehood in abstract matters. But practical reason is concerned with what ought to be done, and this provides real motives. To deny this is to make morality impossible and to reduce humans to sophisticated animals governed by appetite.

Hume's doctrine would make humans into purely passionate beings, governed by appetite and inclination rather than by rational insight into the good. This degrades human nature and makes nonsense of morality. We have the capacity to perceive moral truths—the goodness of benevolence, the wrongness of cruelty—and these perceptions can move us to act rightly even against our passions. Divine law is known through reason, and it obligates us independently of our desires. Hume's philosophy removes the rational foundation of morality, leaving only sentiment and utility. This is skepticism about morality as thorough as his skepticism about causation and the self, and equally unacceptable. Reason is not the slave of passion; it is the divine light by which we discern truth and good.

While I would not endorse Hume's provocative formulation, there is truth in his emphasis on the role of desire in motivation. Reason alone, understood as the faculty that perceives truth and falsity, cannot move the will. The will is moved by good and evil, by what appears desirable or undesirable. However, reason plays a crucial role in determining what we desire by revealing the true nature and consequences of things. Through reason, we can come to desire what is truly good rather than what merely appears good. Thus, while immediate motivation comes from desire, reason governs action by shaping desire. Hume's stark dichotomy between reason and passion misses this interactive relationship.